Fernandeztv,
Appreciate your DGAC post, but I'm not sure that it says what you claim it does. And I'm not saying anything about the thoroughness or appropriateness of the training methods, but about value received. But before we get into details, please think carefully about your question "
...why on earth would an airline invest on a life long copilot? 
That too 50+ of them." If the idea of an entrapped, low-cost labor force doesn't thrill you, then you're thinking like a wannabe pilot, not like an airline executive.
Now to the details.
According to #1, you'll be able to exercise PPL privileges. You'll be able to exercise PPL/IR privileges only in a multi-crew operation. So, no renumeration, and no instrument work unless you have a similarly qualified crewmate, AND the operation requires a multi-crew. And you can exercise PPL privilege as co-pilot of an AIRCRAFT requiring a multi-crew (not the operation). Again, no money.
A PPL gives you the same privileges as 1(i) and 1 (iii). A PPL/IR removes the restriction for multi-crew operations.
How is #2's "demonstrated ability" verified? Checkride? Syllabus completion? Again, you've now matched the same privileges as a PPL/IR.
Number 3 is interesting. Okay, a minimum of an additional 10 hours, or 70 hours depending on your viewpoint. Take the low estimate. So you'll have to fund an additional 10 hours out-of-pocket anyways. Now, how does that additional "acting as PIC under supervision" get accumulated? Remember PF/PNF has nothing to do with PIC. So, at a minimum, you will have to gain the additional 60 hours under supervision in an AIRCRAFT and OPERATION authorized for single-pilot operations.
So you have now equaled the privileges of a CPL/IR, which includes
Act as second-in-command/co-pilot in commercial air transportation in airplanes
required to be operated with a co-pilot by the type certificate of the aircraft or the regulations under which the aircraft will be operated
.
And now to #4. You've conveniently ignored my suggestion that you look at the PIC times required for an ATPL. If there are any shortcut hours (and from the lack of inclusion in the regulation I'm betting there aren't any), the license would have to be endorsed "Does Not Meet ICAO Requirements", along with the limitations. This is the same endorsement many states issue when FE time is counted (generously) toward the required flight time.
So now, you have the privileges of an ATP, restricted to multicrew operations, WITHIN the issuing state.
And a strict reading of all this is even worse. For someone who has never held an CPL, the limitations cannot be removed without meeting the standard requirements for an ATPL. Again, all the same questions raised in number 3 rear their heads.
Once you meet the PIC requirements, then you'll have all the privileges of an ATP.
Again, the MPL may be a fast-track to the right seat of a bright, shiney airliner, but it's definitely the slow boat to an ATPL (except for the 1500 TT, where it's on a par with the JAA frozen ATPL).
Best of luck to you.